The accessory subunit of mtDNA polymerase shares structural homology with aminoacyl-tRNA spothetases: Implications for a dual role as a primer recognition factor and processivity clamp

摘要

The accessory subunit of the heterodimeric mtDNA polymerase (Polr) from Drosophita embryos is re- quired to maintain the structural integrity or catalytic effi- ciency of the holoenzyme. cDNAs for the accessory subunit from Drosophila, man, mouse, and rat have been identificd, and comparative sequence alignment reveals that tbe C. lerminal region of about 120 aa is the most conserved. Furthermore, we demonstrate that the accessory subunit of animal pol r has both sequence and structural similarity with class IIa aminoacyl-tRNA synthetases. Based on sequence Similarity and fold recognition followed by homology model- ingg we have developed a model of the three-dimensional structure of the C-terminal region of the accessory subunit of poly. The model reveals a rare five-stranded beta-sheet sur- rounded by four alpha-helices with structural homology to the anticodon-binding domain of class IIa aminoacyl-tRNA syn- thetases. We postulate that the accessory subunit plays a role in recognition of RNA primers in mtRNA replication, to recruit pol r to the template-primer junction. A ssimilar role is served by the r-complex in Escherichia coli DNA polymerase Ill, and indeed our accessory subunit model shows structural similarity with the N-terminal domain of the delta subunit of the r-complex. Structural similarity is also found with E. coli thioredoxin, the accessory subunit and Processivity factor in hacteriophage T7 DNA polymerase. Thus, we propose that the accessory subunit of poI r is involved both in primer recog- nition and in Processive DNA strand elongation.